Method of manufacturing wheels for motor vehicles

ABSTRACT

The semifinished casting for an alloy road wheel undergoes three successive machining operations in which material is removed by stages to produce an inner circumferential seating, destined ultimately to accommodate one bead of a tire, a center hole coaxial both with the inner seating and with the axis of rotation of the wheel, also a set of fixing holes of which the axes describe a circle centered on the axis of rotation, and finally, an outer circumferential seating destined to accommodate the remaining bead of the tire, which is offset from the axis of rotation in such a way as to achieve a non-uniform distribution of mass, limited to the part of wheel affording the outer circumferential seating, that varies progressively with the change in angular position around the axis of rotation between a maximum and a minimum value.

BACKGROUND OF THE INVENTION

The present invention relates to a method by which to fashion wheels formotor vehicles, in particular cast wheels, and to the wheels obtainableby such a method. More exactly, the invention relates to a method ofmanufacturing a wheel which when fitted with a pneumatic tire will becapable of ensuring a reduction in the forces of inertia generated bythe assembled wheel and tire once set in rotation. It is known that arigid body such as a mechanical component, when rotated about a fixedaxis, becomes invested with centrifugal force and torque. In thespecific instance of road wheels, fitted with tires and secured to therelative supporting members of a motor vehicle, the inertial and torqueforces generated in rotation give rise to serious functionaldisturbances; their effect is to trigger rotational reactions in thesupporting members, whereupon vibrations are set up and transmittedthrough the mechanical components to which the selfsame supportingmembers are mounted. What is more, the vibrations generated in thismanner are clearly discernible through the steering system of thevehicle and, at higher road speeds, liable not only to cause discomfortto the driver but also to create difficulties in maintaining direction,with the result that normal road-holding is adversely affected.

The variations in inertial forces generated in this manner can bemeasured utilizing special equipment designed to give a reading frompolar coordinates centered on the axis of rotation. In particular theoperator can measure variations in inertial forces referred to the tire,exclusively, and to the wheel exclusively, also to the wheel and tiretogether. The problem in question has long been recognized, as witnessedby U.S Pat. No. 1,860,216 which explicitly discloses the notion ofsecuring a sleeve to the wheel disc, disposed with its axis offset fromthe axis of rotation and connected thus to the hub of the vehicle. Theeffect is to generate a greater weight in one half of the wheel than inthe other, the heavier half being that occupied by the valve hole. Thedegree of offset between the axis of the sleeve and the axis of rotationof the assembled wheel and tire is established by determining the extentto which the selected tire registers out of balance. Thereafter, theunbalanced tire is fitted to the unbalanced wheel in such a manner thateach imbalance can compensate the other. In short, the localized massesupsetting the balance respectively of the tire and of the wheel will bedisposed on opposite sides of the axis of rotation to the end ofachieving a mutual compensation.

Mention is made as early as 11-15^(th) Jan. 1971, in a publication byS.A.E, the American Society of Automotive Engineers, of the concept ofreducing, or rather compensating variations in kinetic forces by fittingpneumatic tires to wheels eccentrically. By contrast, the prior artembraces the technique, as stated in the background of U.S Pat. No.3,808,660 for example, of measuring the variations in inertial forcesgenerated by a tire and removing portions of material (rubber) in theevent that such variations exceed a prescribed limit, effecting a seriesof measurements and successive removals of material. In order to avoideffecting numerous measurements and successive removals of material,while ensuring that a balanced tire will not ultimately form part of anunbalanced wheel-tire assembly, U.S. Pat. No. 3,808,660 discloses theprocedure of fitting the tire to the wheel, effecting a measurement asdescribed above and localizing the point at which peak amplitude of thefundamental frequency occurs, then offsetting the center hole from theaxis of rotation by an amount commensurate with the peak value measured.The offset and the heavy portion of the tire are located on oppositesides of the axis of rotation in such a way that the one is cancelledout by the other. It will be seen from the drawings of the patent inquestion that the offset is obtained by the addition of a ring, insertedinto the flanged center hole of the wheel disc and fitted over the hubpivot. The bore and the surface of revolution of the ring are offset bya predetermined amount, which is a function of the peak value asmeasured for the assembled wheel and tire. When mounting the wheel tothe hub, the ring can be rotated on the pivot to find the correctangular position of the offset in relation to that of the pivot; in likemanner, the ring can be repositioned or replaced in the event that theinitial correction should prove inaccurate at a subsequent verification.

Referring again to the prior art, U.S. Pat. No. 3,951,563 addresses thedifficulty in achieving a faultless assembly of the disc and rim of awheel, with the center hole of the disc and the axis of rotationcoinciding to best advantage. The method disclosed is one of setting theassembled wheel in rotation about a reference axis, measuring thevariations in the radial forces generated during rotation, then locatingthe center hole in a position, offset from the axis of rotation, atwhich minimum variation occurs.

Similarly, there are dynamic tests used by motor vehicle manufacturersfor measuring the high and/or the low value of the fundamental frequencyproduced by radial forces of inertia, both in wheels and in tires, andidentifying the corresponding angular position. Having run the tests,the tire is fitted to the wheel such that the position identified as`high` for the one coincides with the position identified as `low` forthe other, a method known as "match mounting" to those skilled in theart. Such a procedure is somewhat lengthy and costly, however,especially when considering that it has to be repeated on each wheel andtire assembled. Given that each motor vehicle has at least four wheels,it requires little imagination to envisage the annual cost ofmatch-mounting wheels and tires for all the vehicles produced in any oneyear. The ongoing demand for improvement in this field has been met onthe one hand by an effort on the part of tire manufacturers to provideincreasingly better balanced products, or at least to provide tiresbearing an indication of the area of maximum or minimum imbalance, i.e.the position reflecting the high or low value of the fundamentalfrequency generated in rotation by radial forces of inertia. On theother hand, the makers of wheels have made similar efforts with theirproducts so that discs also bear these same `high` and `low`indications. Accordingly, it becomes possible for wheels and tires to beassembled correctly without any dynamic balancing operations beingnecessary, but simply by positioning the `high` area of the tire tocoincide with the `low` area of the respective wheel. Given the need toprovide wheels with a `high` or `low` reference mark as described above,and with the end in view of reducing the offset between the center holeand the circle described by the axes of the stud holes, by way of whichthe disc is secured to the hub of a vehicle, U.S Pat. No. 4,279,287discloses a method of locating the `high` or `low` reference at apredetermined angular position of the wheel, in this instance a pressedsteel disc. This patent method envisages the preparation of a rim anddisc assembly, the disc being inserted into and welded to the rim, ofwhich the disc affords at least one center hole disposed offset from theaxes of the circumferential seatings destined to receive the beads ofthe tire. In practical terms the procedure is one of punching the centerhole and the stud holes in a single operation, and in such a way thatthe axis of the former is concentric with the axis of the circledescribed by the latter, but offset in relation to the axis of thedisc-rim assembly overall. The axis of rotation of the wheel is thusdisplaced by translation away from the axes of both circumferentialseatings, and the imbalance built into the wheel is a static imbalance.

Examining these references in detail, it is clear that all relate to thepressed steel disc type of wheel, and therefore to a productiontechnique that is relatively imprecise, not least by reason of thedimensions typically involved.

In every instance, moreover, it emerges that the preferred methodpursued is one of offsetting the center hole and the circle of studholes from the circumferential seatings destined to accommodate the tirebeads, which obviously are coaxial, such that the eccentricity of theaxis of rotation is taken up by the elasticity of the pneumatic tire;needless to say, considerable vibration would be transmitted to thevehicle as a result of any such eccentricity if the tire were rigid.

The object of the present invention is to afford a method of fashioningalloy wheels, of the type cast in molds and then machined to remove anallowance of material, whereby an "imbalance" is incorporated of whichthe effect, in each wheel fitted with a respective pneumatic tire andsecured to the hub of a motor vehicle, is to achieve an improved overallbalance both of the assembled wheel and tire and in the movingmechanical parts of the vehicle with which the wheel is associated.

A further object of the invention is to produce alloy wheels with adeliberate imbalance by employing machine tools of conventional designrather than the special toolage currently required as in the case, forexample, of the patent methods referred to above.

SUMMARY OF THE INVENTION

The stated objects are realized in a method of fashioning wheels formotor vehicles according to the invention, in particular, wheelsobtainable by casting, of which the recognizable components are a rim,exhibiting an external surface of revolution and an internal surface ofrevolution, of which the external surface of revolution affords a pairof circumferential seatings, outer and inner, destined to accommodatethe beads of a relative pneumatic tire, and a disc, exhibiting anexternal face and an internal face and affording at least one centerhole.

In the method disclosed, each casting undergoes a first machiningoperation to remove material at least from the inner circumferentialseating, the internal surface of revolution, the internal face and thecenter hole, effected in relation to a single reference axis coincidingwith the axis of rotation of the wheel, a second machining operation inwhich material is removed to create holes for securing the wheel,effected so that the axes of the holes describe a circle concentric withthe reference axis of the first machining operation, and a thirdmachining operation to remove material at least from the outercircumferential seating, which serves to define a surface of revolutioncentered on an axis of rotation offset from the reference axis of thefirst and second machining operations; in this way, the distribution ofmass around the part of the wheel affording the outer circumferentialseating varies progressively in relation to the reference axis of thefirst and second machining operations, between a minimum and a maximumvalue.

The same stated objects are realized in a wheel for motor vehicles,obtainable by casting and finished by implementation of the stepsoutlined above. Similarly, the recognizable components of such a wheelare a rim, exhibiting an external surface of revolution and an internalsurface of revolution, of which the external surface of revolutionaffords an outer circumferential seating and an inner circumferentialseating destined to accommodate the beads of a pneumatic tire, and adisc, exhibiting an external face and an internal face and affording atleast one center hole surrounded by a set of stud holes in a circularformation. In the wheel according to the invention, the innercircumferential seating and the internal surface of revolution aredefined by surfaces of revolution concentric with the axis of the centerhole whereas the outer circumferential seating is defined by a surfaceof revolution centered on an axis offset from the axis of the centerhole, with the result that the depth of material and therefore thedistribution of mass around the part of the wheel affording the outercircumferential seating varies progressively in relation to the axis ofthe center hole between a minimum and a maximum value.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in detail, by way of example, withthe aid of the accompanying drawings, in which:

FIG. 1 shows a frontal elevation of the wheel according to the presentinvention;

FIGS. 2, 3 and 4 are axial sections illustrating the three machiningoperations which form part of the method according to the presentinvention;

FIG. 5 shows an axial section through the wheel of FIG. 1, in whichcertain dimensions are purposely exaggerated to illustrate how thedistribution of mass around a part of the wheel, limited to the outercircumferential seating, varies progressively between a minimum and amaximum value in relation to the axis of rotation of the wheel;

FIG. 6 shows the wheel of FIG. 5, in smaller scale, fitted with apneumatic tire.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

With reference to the accompanying drawings, the invention relates to amethod of fashioning wheels, and in particular, road wheels 1 cast in ametal alloy. Two elements are recognizable in a wheel 1 of the type inquestion, substantially distinct in appearance but incorporated in asingle monolithic structure precisely by reason of the fact that thewheel is a casting. The two elements are a rim 2 and a disc 3.

The rim 2 is annular in shape and exhibits an external surface ofrevolution 4 and an internal surface of revolution 5. The position ofthe disc 3 in relation to the rim 2 is such that the elements assume abell-like aspect, in respect of which the disc 3 presents an externalface 8 and an internal face 9; the disc also exhibits at least onecenter hole 10.

The external surface of revolution 4 of the rim 2 affords an outercircumferential seating 6 and an inner circumferential seating 7, whichare destined to accommodate the beads 14 of a pneumatic tire 13 (seeFIG. 6).

According to the method disclosed, each wheel 1 undergoes a firstmachining operation to remove material at least from the innercircumferential seating 7, the internal surface of revolution 5, theinternal face 9 and the center hole 10; the machining cuts are effectedin relation to a single reference axis 15 which coincides with the axisof rotation of the wheel 1, as shown in FIG. 2. There then follows asecond machining operation whereby material is removed to create studholes 11 in the wheel 1, located such that their axes 17 (See FIG. 4)define a fixing circle 12 centered on the reference axis 15 of the firstmachining operation as shown in FIG. 3. Finally, the method comprises athird machining operation in which material is removed at least from theouter circumferential seating 6 to establish a surface of revolutioncentered on an axis 16 displaced from the reference axis 15 adopted forthe first and the second machining operations, as illustrated in FIG. 4.In this way, the distribution of mass around the portion of the wheel 1that incorporates the outer circumferential seating 6 is renderednon-uniform, characterized by gradual and infinite variation between aminimum value and a maximum value, as measured in relation to thereference axis 15 of the first and second machining operations.

Observing FIGS. 2, 3 and 4, in which the surfaces machined respectivelyduring the first, second and third operations are shown in bold lines,it will be seen that the wheel 1 affords an outer flange 19 and an innerflange 20 projecting frontally from the opposite extremities of the rim.Prior to the first machining operation, and while still in the as-castcondition, the wheel 1 is aligned by means of a first centralizer 22offered to the inside surface of the outer flange 19; the firstmachining operation then comprises the step of producing a referencesurface 21 on the internal face 9 of the disc 3, destined ultimately tobe breasted with and clamped against a corresponding surface afforded bythe hub (not illustrated) of a motor vehicle. Before the secondmachining operation, the wheel 1 is aligned coaxially around its owncenter hole 10 by means of a second centralizer 23 of which the shank 24is inserted through the hole 10 and the face offered to the referencesurface 21 afforded by the disc; the operation might equally well beeffected by securing the wheel 1 around the inner flange 20, for exampleusing a plurality of teeth (not illustrated), and a centralizer 23 ofwhich the face is offered to the annular surface 27 of the flange 20. Inany event, the centralizer 23 will be disposed faultlessly coaxial withthe axis of rotation of the machine tool and, given that the center hole10 and the reference surface 21 and/or the annular surface 27 wereobtained coaxially by a previous machining operation, the secondoperation will necessarily be executed in coaxial alignment with thefirst. Accordingly, the stud holes 11 are distributed around a fixingcircle 12 coaxial with the center hole 10. Prior to the third machiningoperation, finally, the same second centralizer 23 is displacedmarginally through a path parallel with its own axis in the direction ofthe arrow f. This displacement is illustrated in FIG. 4 by the two axesdenoted 15 and 16, the former being the reference axis for the first andsecond machining operations, which coincides with the two axes of thefirst and second centralizers 22 and 23 when aligned, the latterproviding the reference axis for the third machining operation, whichcoincides with the axis of the second centralizer 23 when offset by adistance denoted 25 from the position occupied during the secondmachining operation. In the finished wheel 1, accordingly, the outercircumferential seating 6 is not coaxial with the inner circumferentialseating 7, neither with the center hole 10, nor with the fixing circle12, nor with the external and internal faces 8 and 9 of the disc 3.

As a result of this calculated misalignment, one has a non-uniformdistribution of mass in relation to the reference axis 15 which islimited to the part of the rim affording the outer circumferentialseating 6, the reference axis 15 being one and the same as the axis ofthe center hole 10, and thus coinciding with the axis of rotation of thewheel. FIG. 5 shows a section through the wheel 1 in which certaindimensions are intentionally exaggerated to illustrate the thickness ordepth 18 of the part of the rim 2 affording the outer circumferentialseating 6, at the maximum value 18a and at the minimum value 18b. Asdiscernible from FIGS. 5 and 6, and as intimated above, this same depth18 reflects the difference that separates the surfaces affordedrespectively by the outer circumferential seating 6 and by the internalsurface of revolution 5, the latter being coaxial with the axis ofrotation 15. For further clarity, a phantom line 29 is added in FIG. 5to indicate the external profile of the wheel 1 in its as-castcondition, that is, prior to the first and third machining operationswhich involve removal of the material compassed between the phantom line29 and the finished profile 30 of the wheel 1. Whilst in reality thevariation in depth 18 is minimal, the non-uniform distribution of massaround the rim, as measured from the reference axis 15 (which coincideswith the axis of rotation of the rim, it will be recalled), issufficient to generate both static and dynamic imbalances in thewheel 1. The causes of such imbalances are illustrated by FIG. 6, inwhich the wheel 1 is shown fitted with a pneumatic tire 13. Alsoindicated are two planes Pe and Pi, disposed perpendicular to thereference axis 15 and passing through the anticipated centers of mass ofthe two zones of the wheel subjected to the first and third machiningoperations. In other words, Pe is the anticipated center of mass of disc3 and Pi is the anticipated center of mass of rim 2 after the machiningsteps. Observing FIG. 5, in effect, it will be seen that the machinedwheel 1 is characterized by two distinct portions with different centersof mass, which are separated by the phantom line denoted 28 andindicated as 1a, to the left of the line 28, and 1b, to the right. Asalready intimated, the portion 1a affording the outer circumferentialseating 6 is out of balance with the rest of the wheel; thus, the centerof mass of this same portion 1a occupies a position different to that ofthe remainder of the wheel, hence different to that of the portiondenoted 1b. In effect, therefore, only the one portion 1a of the wheel 1remains out of balance with the axis of reference and of rotation 15. Itis in the notional planes Pe and Pi that centrifugal forces F1, F2 andF3 are generated when the wheel 1 is set in motion about its axis ofrotation 15: in the one plane Pi, which is the nearer to the innercircumferential seating 7 and therefore to the portion of the wheel inwhich the distribution of mass around the axis of rotation 15 isuniform, the forces F3 are equal and opposite, whereas in the otherplane Pe, which coincides with the outer circumferential seating 6, theopposing forces F1 and F2 are of dissimilar intensity; more exactly, theforce denoted F1 is of greater intensity than that denoted F2 by reasonof the fact that the rim depth 18 in the corresponding halves of thewheel is respectively at its maximum value 18a and its minimum value18b. The difference in intensity of these forces F1 and F2 results in astatic imbalance, first, and a dynamic imbalance thereafter. The dynamicimbalance is due to the center of mass in the corresponding plane Pebeing offset from that of the wheel 1 as a whole, such that theresultant of the relative forces F1 and F2 occurs at a given distancefrom the center of mass of the wheel (not indicated), generating amoment M that tends to deflect the axis of rotation 15.

The distribution of mass around the part of the wheel affording theouter circumferential seating 6 will be determined by the degree ofimbalance it is wished to build into the wheel 1 when fitted with a tire13. The imbalance itself, needless to say, is determined by the measureof the offset 25 between the outer circumferential seating 6 and theremainder of the wheel.

With a wheel 1 purposely "unbalanced" in this way, it becomes possiblefor an assembled wheel and tire and the moving mechanical parts withwhich these are associated, to be "balanced" both statically anddynamically. If, in any event, the wheel 1 is fitted with a type of tire13 other than originally envisaged, the operation of balancing theassembled wheel and tire is markedly simple both in principle and inimplementation, as any resulting imbalance can be corrected at theexternal face of the wheel. The practice currently exists, in fact,especially in Europe, of balancing assembled wheels and tires alreadyfitted to the respective hubs of the motor vehicle, with the precise endin view of simulating normal conditions of operation, one factor inwhich are the dynamic imbalances generated by associated movingmechanical parts; these imbalances can thus be corrected by applyingweights of appropriate value to the external face of the wheel 1,whereas an imbalance incorporated into the internal face of the wheelwould obviously disallow correction by this method.

As to the identification of an area relative to the reference axis 15 atwhich the localization of mass will be greatest, the option exists ofshifting the centralizer 23 from the axis 15 in the direction of a hole26 passing through both the rim 2 and the disc 3, destined ultimately toreceive the valve of the pneumatic tire 13. The valve hole 26 is piercedduring the second machining operation, and can therefore be utilized asa reference both for the third machining operation and for the purposesof fitting the tire 13. No mention is made of the valve hole 26 in theforegoing description of the method according to the invention, since ithas no material role in the machining operations which determine thenon-uniform distribution of mass.

To reiterate the features of the wheel 1 according to the presentinvention, the axis of rotation remains one and the same as thereference axis 15 adopted for machining purposes, coaxial with the wheel1 as a whole, and the distribution of mass around the part of the wheelaffording the outer circumferential seating 6 varies progressivelybetween a minimum value and a maximum value. In other words, thethickness of depth 18 of the rim 2 in the area of the outercircumferential seating 6, varies uniformly from a predetermined minimumvalue 18b to a predetermined maximum value 18a in dependence on theoffset distance 25 between the reference axis 15 and the axis ofrotation 16 for the third machining step. With a tire 13 fitted to thewheel 1, its beads 14 located in the outer and inner circumferentialseatings 6 and 7, the offset 25 will be taken up by continualdeformation within the outer wall of the tire only, i.e. the part of thetire associated with the outer circumferential seating 6; the wallassociated with the inner circumferential seating 7, by contrast, willrotate free of any deformation attributable to the geometry of thewheel. The feature in question is illustrated in FIG. 6, where α denotesthe angle of deformation induced in the tire 13 once fitted to the wheel1, and where it will be seen that the left hand bead 14, as viewed inFIG. 6, is displaced in relation to the right hand bead; more exactly,the left hand bead is distanced from the reference axis 15 at the areaof maximum depth 18a afforded by the rim and the outer circumferentialseating 6, and positioned nearer to the axis 15 at a point diametricallyopposite, coinciding with the area of minimum depth 18b.

Thus, given that tires emerge from manufacture with static and dynamicimbalances, the advantages of the present invention derive from the factthat, by providing a wheel proportioned in such a way as to generateboth static and dynamic imbalances when set in rotation, it becomespossible to obtain a wheel and tire assembly in which the correspondingimbalances, static and dynamic, tend to compensate one another andcancel out.

The facility of obtaining a faultless static and dynamic balance is madepossible by the nature of the machining operations envisaged in themethod according to the invention; it will be recalled in fact thatthese are precision machining procedures performed with conventionaltoolage and therefore involving no additional costs.

What is claimed is:
 1. A method of producing wheels for motor vehicles,comprising the steps of;producing a semifinished wheel casting of whichthe recognizable components are a rim, having an external surface ofrevolution and an internal surface of revolution of which the externalsurface of revolution includes an outer circumferential seating and aninner cirumferential seating to accommodate the beads of a pneumatictire, and a disc, having an external face and an internal face andincluding at least one center hole; performing a first machiningoperation to remove material at least from the inner circumferentialseating, from the internal surface of revolution, from the internal faceand from the center hole, effected in relation to a single referenceaxis coinciding with the axis of rotation of the wheel; performing asecond machining operation in which material is removed to create holesfor securing the wheel, effected in such a way that the axes of theholes define a circle concentric with the reference axis of the firstmachining operation; performing a third machining operation to removematerial at least from the outer circumferential seating in such a wayas to define a surface of revolution centered on an axis of rotationoffset from the reference axis of the first and second machiningoperations, with the result that the thickness around the part of thewheel including the outer circumferential seating varies from apredetermined minimum value to a predetermined maximum value independence on an offset distance between the reference axis and the axisof rotation for the third machining step.